The invention relates to a device and technique for measuring the topography of a cornea, and more specifically to single-direction incremental displacement or modulation of a cross grating pattern on a phase plate to obtain simultaneous phase shearing in orthogonal x and y directions, imaging the resulting composite diffraction pattern, and operating on the resulting data to compute the height of each pixel of the cornea and store and display a three-dimensional image thereof.
Various corneal topography measurement devices are available. For example, a commonly used TOMEY automatic keratoscope includes an automatic ring detection algorithm and produces a "power" or diopter map, rather than a topographic map of the cornea. (A diopter is the inverse of the radius of curvature in meters.) Eyses Corporation, of Houston, Tex. makes a simplified video keratoscope which operates on the same principles as the TOMEY device, measuring distortion of projected illumination rings by the cornea to extract slope information that is converted to a diopter map. Both of the devices have the disadvantages that the "maps" produced do not cover the full cornea, and furthermore they fail to actually measure the center region of the cornea, at which accurate measurements are most critical. The "shadows" of the rings contain distortion by non-spherical features of the cornea, but are difficult to digitize over the range of the cornea, especially in the central areas. Lateral resolution of points of the points of the diopter maps are not as accurate as desirable. The present assignee has developed an experimental fringe interpretation cornea topography measurement device in which a return beam reflected by the cornea is split, and phase shearing operations are separately performed in the x and y directions. The results are separately imaged by a photosensitive array and the resulting data then is used to compute the relative phase and the relative height of the cornea at each pixel. This technique has the main disadvantage that it is much slower than desired, and is subject to inherent mechanical inaccuracies involved in shifting the x and y phase plates in separate operations.
There currently is an unmet need for an improved automatic keratoscope which provides an accurate topography of the full cornea, especially the center portions. There is an unmet need for accurately producing a true topography map, rather than the conventional "power" maps or diopter change maps of the cornea.